This invention is directed to an improved high voltage flyback transformer for use in display devices such as television receivers and video monitors.
Conventional flyback transformers include a primary winding which receives horizontal scan rate pulses, and a secondary across which high voltage output pulses are developed. The output pulses are typically rectified to develop the high voltage required by a cathode ray tube.
In many cases, the transformer's secondary comprises four windings, each coupled to an adjacent winding by a diode as shown in FIG. 1, to which reference is now made. The illustrated transformer includes a primary winding 10 which receives horizontal scan rate pulses. The secondary includes windings 12, 14, 16, and 18 which are tuned to the ninth harmonic of the horizontal retrace frequency and coupled between ground and an output 20. A capacitor 22 which is shown coupled to the transformer's output represents the capacity of a cathode ray tube which normally receives the rectified high voltage developed by the transformer.
Diodes 24, 26, 28 and 30 are coupled between the various secondary windings as shown to provide a rectified voltage at the output 20.
Transformers such as those shown in FIG. 1 are preferably designed to provide a reasonably well regulated output voltage, particularly at low levels of anode current in the cathode ray tube. Toward this end, it is known that improved regulation results when the high voltage pulse developed by the transformer's secondary has a relatively flat top such as that shown by waveform B of FIG. 2. By providing such a flat top, the diodes can conduct for a longer interval of time and thereby provide the transformer with a lower output impedance. By way of contrast, waveform A of FIG. 2 illustrates the pulse output of a transformer that does not include provision for flattening the top of the output pulse.
The transformer of FIG. 1 is typical of those which generate a flat topped output pulse by including a tank circuit 32 in series with the primary winding 10. Although the tank circuit 32 provides the desired results, its inclusion is clearly undesirable from a cost standpoint.
Another conventional scheme for developing a flat-topped output pulse is to include a resistor between ground and the output 20 instead of using the tank circuit. This method also works, but the required resistor must be able to dissipate several watts. Again, the cost of the transformer is raised.
In certain applications, a focusing voltage for the cathode ray tube is obtained at a point 34 on the secondary. With the illustrated type of transformer the focusing volgage must ordinarily be filtered by including a high voltage capacitor between the point 34 and ground. Once again, the cost of the transformer is increased.
From the foregoing discussion, it can be seen that conventional flyback transformers require circuit components over and above the transformer itself in order to provide the desired outputs. Consequently, the cost of the transformer is substantially increased beyond what is desirable.
Accordingly, it is a general object of the invention to provide an improved high voltage flyback transformer.
It is a more specific object of the invention to provide a well regulated flyback transformer which is less costly than conventional transformers and which is preferably adapted to provide a focusing voltage without the need for extra filtering components.